This invention relates to an improved fluidized bed system and process therefor.
Fluidized bed processes are used commercially for the ore roasting or refining (such as chlorination of titanium containing materials) combustion of solid carbonaceous material such as coal, hydrocarbon conversion processes (e.g., fluid catalytic cracking), etc.
In such processes, particulate material and gas are fed to a reactor where suitable temperatures and pressures are maintained. The flow rates are adjusted so that the particulate material becomes fluidized, i.e., it is maintained in a state of suspension and has the appearance of boiling.
A good example of a commercial fluidized bed process is that employed for chlorinating titanium containing material. In such processes, particulate coke, particulate titanium containing material, chlorine, and optionally oxygen or air are fed into a reactor at suitable reaction temperature and pressure. Suitable gas flow rates sustain bed fluidization. Gaseous titanium tetrachloride and other metal chlorides are produced and exit the reactor. The titanium tetrachloride can then be separated and used to produce titanium dioxide pigment or titanium metal.
U.S. Pat. No. 2,701,179 discloses a commercial fluidized bed process for chlorinating titanium containing material wherein the particulates are heated by direct contact with reaction product gases, separated in a cyclone and subsequently fed pneumatically with chlorine into the bottom of the fluidized bed reactor.
A problem, however, which has not been solved in the foregoing fluidized bed processes, is that fine particulate material tends to become entrained in the hot gases discharged from the fluidized bed reactor. As a consequence, fine particulates have a short residence time in the reactor and tend to exit unreacted, even when they are recycled. The unreacted fines, therefore, represent not only a disposal problem but also a waste of fuel and/or metallic values in the feed materials.
The fine particles typically are present due to the attrition and degradation of larger feed particles. However, it is also desirable to include fine particulate material in the feed to the process since such materials are often abundant and less expensive than material of larger particle size. For example, there are large quantities of relatively inexpensive titanium containing ore which cannot presently be economically processed because they exist as fine grained sand. For example, the article entitled "Fluidized Bed Chlorination of Rutile" by J. Glasser and W. L. Robinson appeared in the September 9, 1962 publication of the Society of Mining Engineers of AIME, describes a commercial fluidized bed process for chlorinating titanium ore wherein the ore fed to the top of the fluidized bed reactor is of a particle size greater than about 70 microns.
There are several procedures disclosed in the prior art for controlling the flow of solids into fluidized beds. For example, U.S. Pat. No. 4,774,299 discloses a system and process for introducing catalytic powder into a fluidized bed polymerization reactor comprising: (1) a storage vessel for the powder, provided with a feed line and a shut-off valve, (2) a metering device for delivery of powder to an intermediate chamber below it, the chamber having a tube and shut-off valve in its upper part to introduce an inert carrier gas, and (3) conveyor piping having an elbow or bend, a horizontal or substantially horizontal portion and a rapid-opening valve to convey the metered powder to a fluidized-bed reactor. The purpose of the intermediate chamber is to reduce the compactness of the catalyst powder. The catalyst powder can have a mean diameter by mass of between 100 and 400 microns.
U.S. Pat. No. 3,850,582 describes an apparatus for adding fresh make-up catalyst to a process unit, e.g., a fluidized catalytic cracking unit. The apparatus comprises: (a) a main fresh catalyst hopper; (b) a catalyst metering hopper, which is essentially a vertical standpipe, containing a fluidized bed of catalyst; (c) a catalyst transfer line connecting the main hopper to the metering hopper; (d) a valve to control withdrawal of catalyst from the metering hopper for transfer to the process unit; and (e) means for measuring and recording the pressure differential across the catalyst fluid bed in the metering hopper, to monitor the rate of addition of catalyst to the process unit.
The catalyst hopper is provided with a cyclone separator from which the gas separated from solids is passed via a conduit to the main catalyst hopper. Pressure corrections for the pressure drop across the valve controlling catalyst withdrawal from the metering hopper is performed by opening and closing a valve in a vent line from the main catalyst hopper.
U.S. Pat. No. 3,105,736 discloses an improved method of feeding particulate solids into a reactor producing effluent gases whereby these gases are prevented from penetrating the feeding system. The method applies particularly to the preparation of metal halides, e.g., TiCl.sub.4, where particulate metal ore and coke are reacted in a fluidized bed with halogen gas. The improvement comprises establishing a pair of fluidized beds of the feed material which are connected below the beds. Both of these beds are fluidized with inert gas. The upper surface of the first bed is open to the solids feed, and that of the second bed communicates directly with the reactor. Feeding material to the first bed causes material to flow into the second bed and from there into the reactor. The gas pressure in the second bed is maintained above the reactor pressure, preventing reaction effluent gases from penetrating the feed system.
U.S. Pat. No. 2,905,635 discloses a method for hydrocarbon conversion in presence of a fluidized catalyst, which comprises: (a) mixing hydrocarbons and catalyst; (b) conveying this mixture through a transfer pipe into a fluidized bed reactor; (c) reacting the hydrocarbon, and removing spent catalyst into a transfer pipe below the reactor; (d) withdrawing spent catalyst through a valve into a dense phase fluidized bed in a vertical standpipe; (e) removing spent catalyst through a valve at the bottom of the standpipe, and transporting it with gas to a regenerator; (f) withdrawing regenerated catalyst, through a valve below the regenerator, into a dense phase fluidized bed in a vertical standpipe; and (g) removing regenerated catalyst through a valve at the bottom of the standpipe to be mixed with hydrocarbons in step (a).
The flow of catalyst into and out of the reactor and standpipes is controlled with valves connected to differential pressure controllers. The differential pressure controllers measure the pressures across the valves which change with the bed levels above the valves.
U.S. Pat. No. 2,892,773 describes a cyclical fluidized bed process and apparatus in which fluidized solid particles, suspended in a first reaction gas in a dense phase, are circulated continuously from a hopper through a reaction vessel and back to the hopper where the first reaction gas is separated from the solid particles. Fluidized solid particles are cycled between the hopper and a second reaction vessel, in which the particles are suspended in a second reaction gas. Particle cycling is performed by the use of pressure regulating valves, together with a differential pressure controller and a timer, to change periodically the direction of the pressure difference between the hopper and the second reaction vessel.
For example, a gas exit line with a pressure control valve communicates with the second reaction vessel via a separator. The timer actuates the valve which increases the pressure in the second reaction vessel ultimately changing the direction of the pressure differential between the first and second reaction vessels. The change in pressure differential causes the timer to open a slide valve thereby permitting particles to travel from the second reaction vessel to the first via a transfer line.
U.S. Pat. No. 2,881,133 describes a system in which particulate solids are circulated between two or more different fluid-bed treating zones, and no mixing between the gases in the different zones is permissible. The system is characterized by the following features: (a) a vertical fluidized solids standpipe section which provides a pressure buildup; (b) a sharp bend at the bottom of the standpipe; (c) a slanted riser side sloped at about 60 degrees from the horizontal; (d) a large radius bend connecting the slanted riser with a vertical riser; and (e) usually a vertical riser leading directly into a receiving vessel or into a transfer line sloping downward into such a vessel.
The combination of a vertical standpipe, a sharp bend, and a slanted riser as in (a),(b),(c) is sometimes called a J-bend. Fluidization is maintained in the J-bend by adding inert gas at several points.